The present invention concerns an amplifier circuit comprising at least:                an amplifier unit arranged to receive an input signal and to influence the amplification of this input signal and to transmit an output signal which is intended to lie at a desired level, and        a control unit arranged to sense said output signal and to deliver a first control signal for controlling the amplification of said amplifier unit.        
U.S. Pat. No. 5,917,639 and U.S. Pat. No. 6,055,094 describe different kinds of known amplifier circuits.
An amplifier circuit may for example be used when it is desired that a signal should lie at a predetermined level. Such an amplifier circuit is often called AGC (Automatic Gain Control).
FIG. 1 shows an example of an already known circuit. The circuit comprises an amplifier unit 201. The amplifier unit receives an input signal at an input 202 and transmits an amplified output signal via an output 203. The level of the output signal is sensed by a level detector 211 which delivers an output signal which represents this level. The output signal from the level detector is conducted to an integrator unit 212 which integrates the difference between the output signal from the level detector 211 and a desired value, here represented by an input 214 of the integrator unit 212. The integrator unit 212 has an output 219 for the integrated signal. This integrated signal is conducted to a second input 204 of the amplifier unit 201. The signal at this second input 204 controls the amplification of the amplifier unit 201.
The amplifier unit 201 may for example be arranged such that a higher value of the signal at the second input 204 means a lower amplification. Of course also the opposite relationship is conceivable. If it is assumed that the output signal from the amplifier unit 201 lies higher than the predetermined level, the signal from the level detector 211 to the integrator unit 212 thus lies higher than the desired value 214. This means that the value of the signal at the output 219 of the integrator unit 212 increases, which means that the amplification of the amplifier unit 201 is reduced. The circuit according to FIG. 1 thus ensures that the output signal from the amplifier unit 201 lies at a predetermined level.
A disadvantage with prior known amplifier circuits is that it is often difficult to control the amplification for different kinds of signals. For example, some signals may comprise pulses of a very high frequency and other signals may comprise pulses with relatively long pauses between the pulses. When pulses arrive with long pauses between the pulses, the amplifier circuit may tend to amplify the signal, which may mean that when then pulses of a high frequency arrive, the amplification may be too high, which may lead to different problems, the amplifier may for example be saturated and the amplification may become non-linear.